1. Field of the Invention
This invention relates to a synchronizing signal recovery circuit, and it relates particularly to such a circuit which is useful for extremely noisy data signal channels such as are found in some mobile radiotelephone systems.
2. Description of the Prior Art
Prior mobile radiotelephone systems generally assign a channel for a particular call connection, and that same radio channel is utilized throughout the call even though the radio portion of the call connection may be completed through different base stations as a mobile unit moves from one place to another within a given mobile service area. The system operates on a noisy radio channel; but a low data communication rate, e.g., up to a few hundred hertz, for controlling the radio call connection is usually adequate to handle the necessary data communication since there is relatively low data communication volume. In addition there is ample time available to recover synchronizing signals by state of the art techniques, which will be hereinafter further considered.
In newer high-capacity radiotelephone systems, a small-cell configuration is employed in which a given set of channels can be simultaneously reused many times in a single urban mobile service area by properly proportioning radiated signal powers in relation to cell size and by handing off a mobile unit from channel to channel as it moves from cell to cell in the mobile service area. Such systems are described in a number of source publications; and several examples include U.S. Pat. Nos. 3,663,762 to A. E. Joel, Jr., and 3,819,872 to H. E. Hamrick, and "High Capacity Mobile Telephone System - Technical Report" of December 1971, prepared by Bell Laboratories, and filed with the Federal Communications Commission in the same year under Docket 18262. Copies of the latter report may be currently purchased from Downtown Copy Center, 1730 K Street, N.W., Washington, D. C. 20006.
Details of such high capacity radiotelephone systems are not essential to an understanding of the present invention beyond a comprehension that such systems work on noisy Rayleigh fading channels and require a high data transmission bit rate in order to effect mobile unit location and handoff between channels on a real time basis. Such operation must be carried out without significantly disturbing subscribers who are active on a call connection. In addition, the indicated type of operation is necessary to realize the advantage of relatively high traffic capacity in terms of numbers of simultaneously operative call connections with a relatively few frequency channels, as compared to prior mobile radiotelephone systems which are incapable of simultaneously reusing frequency channels within a single mobile service area. In one such high capacity mobile radiotelephone system, paging channels include continuous data transmission from base stations at a 10-kilobit per second rate that is extremely stable. Noise on the radio channels gives the data the appearance of instability. During communication on voice channels, data is sent to an individual mobile unit by briefly blanking voice transmission and sending a burst of 10-kilobit per second data during the blanking interval, which interval is so short that the subscriber hears only a click.
Substantial difficulty is encountered in mobile units in high-capacity, small-cell systems of the type described in recovering synchronizing signals from the received data signals in order that the received data may be utilized by the mobile unit controller. These difficulties arise because, for example, the noise, including Rayleigh fading, is so severe that there is substantial jitter in leading and trailing edges of data pulses and data bits are frequently severely broken so that, in terms of signal transitions, a single data bit may appear as several bits. Rayleigh fading is experienced in a moving mobile unit and is signal variation due to addition and cancellation of reflected waves. Such fading occurs at intervals as low as 7 to 14 inches at 850 MHz in the frequency band in which the mentioned high-capacity, small-cell system is currently designed to operate. In such a noisy signal environment it is difficult to extract bit and word synchronizing signals in any event. It is particularly difficult to do it with the high speed that is required to respond to the data on a real time basis to hold synchronization without causing intolerably high data error rates and to perform all of this on an economical basis so that the cost of mobile units is not so high as to discourage the use of the high-capacity small-cell radiotelephone technique.
One example of a problem encountered in recovering bit synchronization signals lies in the fact that analog phase-locked loops, which are frequently employed for this purpose in prior art systems, suffer a substantial conflict in their operating characteristics. Thus, a narrowband type of operation is necessary to limit the range of signal frequencies to which the circuit can lock in order to reduce the likelihood that it will lock onto noise frequency signals. However, another characteristic of phase-locked loops is that it is advantageous for them to have fast pull-in response so that they can be quickly initialized. The fast pull-in is usually not available in narrowband phase-locked loops because the narrower the band of the operation the slower is the pull-in operation. In addition to the aforementioned conflict, temperature conditions experienced by a mobile unit can vary widely, and phase-locked loops are notoriously sensitive to temperature variations. Some of the best commercially available phase-locked loops exhibit a temperature dependent sensitivity of only about 250 parts per million per degree Centigrade. Such a characteristic will cause the free-running voltage controlled oscillator of the loop in a mobile radio environment to drift as far away as 250 Hertz or more from the nominal frequency. This in turn forces the designer to adjust the noise bandwidth to be much wider than the acceptable value needed to cope with the high noise condition on the channel. In other words, the phase-locked loop must be designed to capture over at least the range that can be spanned for its temperature sensitivity; but then the loop responds to a correspondingly wide noise bandwidth and exhibits excessive jitter.
On the question of recovering word synchronizing signals, mobile units experience all of the problems of bit synchronizing signal recovery as well as experiencing additional recovery problems. For example, the possible presence of voice and/or noise in the received signal at the mobile unit causes a high probability that a false synchronizing character will be generated. Consequently, prior art circuits that simply recognize such a character are usually inadequate to operate in this noisy environment and can permit intolerably high rates of false synchronization with the consequent loss of data.